Infants and other incontinent individuals wear disposable absorbent articles such as diapers to receive and contain urine and other body exudates. Training pants or pull-on diapers have become popular for use on children able to walk and often who are toilet training. Many disposable pull-on garments use elastic elements secured in an elastically contractible condition in the waist and/or leg openings. Typically, in order to insure full elastic fit about the leg and the waist such as is provided with durable undergarments, the leg openings and waist opening are encircled at least in part with elasticized bands of rubber or other materials positioned along the periphery of the respective opening.
Stretchable laminates structures used in the chassis of absorbent products and known in the art are for the great majority constructed by using prestrained elastic materials that are adhesively bonded onto two standard nonwoven layers. The elastic material may be, for example, strands, strips of elastic film, or tapes of elastic film. This is often referred to as “live stretch”. When fully strained, the elastic-based structures may need to provide stretch up to 120% or to 200%, depending on location, for example, if used in stretch back ears in taped products or in side panels in pant products. However when used as a chassis component that circles around the body and provides 360 degree all-around stretch, 80-170% stretch can suffice. In order to achieve this amount of stretch, the elastic, such as strands or tapes of elastic film, must be prestrained to a higher amount before the nonwoven is bonded onto them. As the nonwovens are applied onto the prestrained strands, the nonwovens form gathered structures as they are forced to accommodate the recovery of the strands. This produces an expansion of the laminates in the direction perpendicular to the plane defined by the strands. The above phenomenon is often referred to as gathering or puckering of the web. The more bonding of the strands and the webs together, the more this constriction takes place during the recovery process.
While these structures and the resulting puckering have been generally accepted by consumers short of alternatives, it has become clear that from a performance standpoint, large amounts of puckering have an undesirable effect to the touch and feel of the final laminates. It also makes it very difficult to create a garment-like look when used in a chassis, which is an ever more desirable feature sought by the consumers.
From a cost standpoint, there are also several negatives: (i) as the nonwovens gather, their basis weight increases and therefore the nonwoven cost increases. For instance, for nonwoven layers applied onto 150% pre-stretched strands or tapes, there is a 250% basis weight increase of the nonwoven layers concomitant with a 250% increase in cost. So 12 grams per square meter (gsm) becomes 30 gsm after gathering with a concomitant increase in cost; (ii) bonding nonwovens together along with elastic strands or tapes strapped in between is extremely inefficient and requires a large amount of adhesive as a lot of adhesive diffuses into the nonwoven itself and becomes ineffective. About 16-20 gsm of adhesive is generally needed; (iii) printing on a nonwoven is neither easy nor cost-effective. Moreover, given the highly irregular puckered state in the final stretch laminates, it is virtually impossible to have attractive and sharply defined patterns of any kind printed onto them.
From a process standpoint, the more elastic strands or tapes are used, the more elasticity can be imparted to the stretch laminate and/or the more finely the strands or tapes can be distributed; but the more costly the laminate and the more complex the process becomes with a greater risk of strand or tape failure during construction and the issues of process reliability that result from it. There is a clear tradeoff between a fine dispersion of lower diameter strands and the increase in the occurrence of strand failure. Also, depositing large amounts of adhesive is always a challenge. Finally, thermal bonding of the laminates onto itself where seaming is needed to construct the product is difficult at high speed due to the high melting temperature of the nonwovens, and possibility of strand breakage. Thermal bonding also results in creep of strands in stretched laminates, i.e. strands coming lose and retracting.
As an alternative to the kind of “live stretch” produced by strand-based laminates, elastic film-based laminates have also been disclosed and used in so-called “zero-strain” structures. Examples of those are disclosed in US Patent Publications 2007/0287348 and 2008/0045917. In these, nonwovens may be bonded onto an elastic film and the laminate is then subjected to an activation process that unlocks the constraints imposed by the nonwoven and frees up the ability of the film to stretch and recover. These produce laminates structures which are very appealing for the look and feel and are ideal when introduced as stretchable outercovers in disposable absorbent products. However, in order for them to be effective, one might have to include high basis weight and somewhat costly elastic films, or they might offer only a limited range of stretch. Even if the high level of strain is achieved via use of higher film basis weight, the activated laminates do not look aesthetically appealing because of very defined corn-row like appearance, and somewhat broken nonwoven.
In view of all these issues, it is of great interest to create a different technology that can impart the high levels of stretch performance obtained in strands-based laminates, along with the process reliability and attractive final attributes of elastic film-based ones.